1. Field of the Invention
The present invention relates to a valve driving device for an internal combustion engine, and more particularly to a valve driving device for an internal combustion engine in which an oscillating member is interposed between a cam and a valve to synchronize the oscillation of the valve with the rotation of a camshaft.
2. Description of the Related Art
A conventional variable valve driving device for an internal combustion engine is described in, for example, Japanese Patent Application Publication No. JP-A-2004-521235, that mechanically changes the duration and lift of a valve. This type of conventional variable valve driving device includes a return spring (torsion coil spring) that impels a pivot lever (oscillating member), interposed between a cam and a valve, against the cam.
The variations in the shape of the torsion springs are inherent in the production process. In the variable valve driving device of the above related art, the torsion spring impels the pivot lever to oscillate within a predefined oscillating range. Therefore, such variations in shape of torsion springs can cause some torsion springs, incorporated in the valve driving device, to generate a greater impelling force, and others to generate a smaller impelling force, than the target value.
As a result, the torsion springs with an excessively large impelling force may not be able to satisfy the allowable stress for the required fatigue strength, thus providing only reduced durability. On the other hand, those torsion springs with an excessively small spring force may not be able to provide sufficient impelling force for the inertial force of valve driving components (such as the above oscillating member) when the engine speed is high. Such an insufficient spring force at high engine speed does not permit the valve driving components to maintain synchronous operation with rotation of the cam (i.e., a so-called jump occurs), thus requiring the allowable speed of the internal combustion engine to be reduced.
In the conventional variable valve driving device described above, the length of one arm of the torsion spring for contacting the pivot lever is increased to reduce its spring constant. According to such an approach, changes in spring force due to variations in shape of torsion springs can be reduced. However, increasing the size of the torsion spring, impairs the mountability of the variable valve driving device to the internal combustion engine.